The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Generally, a local oscillator (LO) generates a periodic signal, referred to as a clock, that is used in many electronic circuits including wireless transceivers, data converters, wireline and optical serial data communication links, processors, and memory circuits. A voltage-controlled oscillator (VCO) locked to a reference clock by a phase-locked loop (PLL) is often used to generate the clock on chip. In some circuits, however, the VCO output cannot be directly used as clock because some power may leak from a power amplifier to the VCO, and if the VCO and the power amplifier operate at similar frequencies, the VCO frequency may be pulled away from its center frequency towards the power amplifier frequency, which is referred to as power amplifier pulling. Further, clocks of different frequencies may be needed to support different communication standards in a single communication device. Accordingly, conventional clock generation schemes are insufficient for generating clocks at some of the required different frequencies.